1. Field of the Invention
The present invention broadly relates to novel methods for making complex oxidation reaction products such as perovskite bodies, including methods for making shaped complex oxidation reaction products. This invention can be used to make superconducting articles.
2. Background and Commonly Owned Patent Applications
Compounds such as ceramics are conventionally formed by a variety of methods. These methods involve the following general steps: (1) preparation of material in powder form; (2) grinding or milling of powders to obtain very fine particles; (3) formation of the powders into a body having the desired geometry, such as by uniaxial pressing, isostatic pressing, injection molding, tape casting, and slipcasting (with allowance for shrinkage during subsequent processing); (4) densification of the body by heating it at an elevated temperature such that the individual powder particles merge together to form a coherent structure (either without the application of pressure, or through the application of external pressure either uniaxially or isostatically); and (5) finishing, frequently by diamond grinding, as required. Finishing operations are often difficult and expensive to perform, and, in some cases, much of the expense of conventional shaped ceramics is attributable to the cost of finishing operations.
The assignee of the present application has recently discovered new methods for forming ceramics by directed oxidation of a suitable bulk precursor metal. These methods are disclosed generically in copending and Commonly Owned U.S. Pat. application Ser. No. 818,943, filed Jan. 15, 1986, now U.S. Pat. No. 4,713,360, issued on Dec. 15, 1987, in the name of Marc S. Newkirk et al., and entitled "Novel Ceramic Materials and Methods for Making the Same". This application discloses a novel method for producing a self-supporting ceramic body by oxidation of a precursor molten metal or parent metal to form an oxidation reaction product. More specifically, the parent metal is heated to an elevated temperature above its melting point but below the melting point of the oxidation reaction product in order to form a body of molten parent metal which reacts upon contact with a vapor-phase oxidant to form the oxidation reaction product. The oxidation reaction product, or at least a portion thereof which is in contact with and extends between the body of molten parent metal and the oxidant, is maintained at the elevated temperature, and molten metal is drawn through the polycrystalline oxidation reaction product and towards the oxidant, and the transported molten metal forms oxidation reaction product upon contact with the oxidant. As the process continues, additional metal is transported through the polycrystalline oxidation reaction product thereby continually growing a ceramic structure of interconnected crystallites. Usually, the resulting ceramic body will contain therein inclusions of non-oxidized constituents of the parent metal drawn through the polycrystalline material and solidifed therein as the ceramic body is cooled after termination of the growth process. As explained in this Commonly Owned Patent Application, resultant novel ceramic materials are produced by the oxidation reaction between a parent metal and a vapor-phase oxidant, i.e., a vaporized or normally gaseous material, which provides an oxidizing atmosphere. In the case of an oxide as the oxidation reaction product, oxygen or gas mixtures containing oxygen (including air) are suitable oxidants, with air usually being preferred for obvious reasons of economy. However, oxidation is used in its broad sense in the Commonly Owned Patent Application and in this application, and refers to the loss or sharing of electrons by a metal to an oxidant which may be one or more elements and/or compounds. Accordingly, elements other than oxygen may serve as the oxidant, e.g. nitrogen.
Commonly Owned Patent Application U.S. Ser. No. 819,397, filed Jan. 17, 1986, now U.S. Pat. No. 4,851,375, issued on July 25, 1989, discloses related methods for forming composite ceramic bodies. As described in this application, a ceramic composite is formed by placing a permeable mass or aggregate of a substantially inert or nonreactive filler material adjacent to or in contact with the parent metal such that the growing oxidation reaction product of the parent metal infiltrates and embeds at least a portion of the filler material. The present metal is heated as described above, and the oxidation reaction of the parent metal and vapor-phase oxidant is continued for a time sufficient for the oxidation reaction product to grow through or infiltrate at least a portion of the filler material, thus producing a composite body having a ceramic matrix of oxidation reaction product embedding the filler material, such matrix also optionally containing one or more metallic constituents.
Commonly Owned Patent Application U.S. Ser. No. 338,471, which is a continuation of U.S. patent application Ser. No. 861,025, filed May 8, 1986, discloses particularly effective methods in which the filler is formed into a preform with a shape corresponding to the desired geometry of the final composite product. The preform may be prepared by any of a wide range of conventional ceramic body formation methods (such as uniaxial pressing, isostatic pressing, slipcasting, sedimentation casting, tape casting, injection molding, etc.) depending largely on the characteristics of the filler. Initial binding of the particles prior to infiltration may be obtained through partial sintering or by use of various organic or inorganic binder materials which do not interfere with the process or contribute undesirable by-products to the finished material. The preform is manufactured to have sufficient shape integrity and green strength, and should be permeable to the growth of oxidation reaction product, preferably having a porosity of between about 5 and 90% by volume and more preferably between about 25 to 75% by volume. Also, an admixture of filler materials and mesh sizes may be used. The preform is then contacted with molten parent metal on one or more of its surfaces for a time sufficient to complete growth and infiltration of the preform to its surface boundaries.
As disclosed in Commonly Owned Patent Application U.S. Ser. No. 861,024, filed May 8, 1986, now allowed a barrier means may be used in conjunction with the filler material or preform to inhibit growth or development of the oxidation reaction product beyond the barrier. Suitable barrier means may be any material, compound, element, composition, or the like, which, under the process conditions of this invention, maintains some integrity, is not volatile, and preferably is permeable to the vapor-phase oxidant while being capable of locally inhibiting, poisoning, stopping, interfering with preventing, or the like, continued growth of oxidation reaction product. Suitable barriers for use with aluminum parent metal and oxygen as the oxidant include calcium sulfate (plaster of paris), calcium silicate, and Portland cement, and mixtures thereof, which typically are applied as a slurry or paste to the surface of the mass of filler material. These barrier means also may include a suitable combustible or volatile material that is eliminated on heating, or a material which decomposes on heating, in order to increase the porosity and permeability of the barrier means. Still further, the barrier means may include a suitable refractory particulate to reduce any possible shrinkage or cracking which otherwise may occur during the process. Such a particulate having substantially the same coefficient of expansion as that of the filler bed or preform is especially desirable. For example, if the preform comprises alumina and the resulting ceramic comprises alumina, the barrier may be admixed with alumina particulate, desirably having a mesh size of about 20-1000, but may be still finer. Other suitable barriers include refractory ceramics or metal sheaths which are open on at least one end to permit a vapor-phase oxidant to permeate the bed and contact the molten parent metal. As a result of using a preform, especially in combination with a barrier means, a net shape is achieved, thus minimizing or eliminating expensive final machining, grinding or finishing operations.
Commonly Owned Patent Applications U.S. Ser. No. 823,542, filed Jan. 27, 1986, now U.S. Pat. No. 4,828,785, issued May 9, 1989 and U.S. Ser. No. 896,157, filed Aug. 13, 1986, now allowed disclose methods for reliably producting cavity-containing ceramic bodies of a size and thickness which are difficult or impossible to duplicate with previously available technology. Briefly, the inventions therein described involve embedding a shaped parent metal precursor in a conformable filler, and infiltrating the filler with a ceramic matrix obtained by oxidation of the parent metal to form a polycrystalline oxidation reaction product of said parent metal with an oxidant and, optionally, one or more metallic constituents. More particularly, in practicing the invention, the parent metal is shaped to provide a pattern, and then is emplaced in or surrounded by a conformable filler which inversely replicates the geometry of the shaped parent metal. In this method, the filler (1) is permeable to the oxidant when required as in the case where the oxidant is a vapor-phase oxidant and, in any case, is permeable to infiltration by the developing oxidation reaction product; (2) has sufficient conformability over the heat-up temperature interval to accommodate the differential thermal expansion between the filler and the parent metal plus any melting-point volume change of the metal; and (3) when required, at least in a support zone thereof enveloping the pattern, is intrinsically self-bonding, whereby said filler has sufficient cohesive strength to retain the inversely replicated geometry with the bed upon migration of the parent metal as described below. The surrounded or emplaced shaped parent metal is heated to a temperature region above its melting point but below the melting point of the oxidation reaction product to form a molten parent metal. The molten parent metal is reacted in that temperature region or interval with the oxidant to form the oxidation reaction product. At least a portion of the oxidation reaction product is maintained in that temperature region and in contact with and between the body of molten metal and the oxidant, whereby molten metal is progressively drawn from the body of molten metal through the oxidation reaction product, concurrently forming the cavity as oxidation reaction product continues to form within the bed of filler at the interface between the oxidant and previously formed oxidation reaction product. This reaction is continued in that temperature region for a time sufficient to at least partially embed the filler with the oxidation reaction product by growth of the latter to form the composite body having the aforesaid cavity therein. Finally, the resulting self-supporting composite body is separated from excess filler, if any.
The entire disclosures of each of the Commonly Owned patent applications identified above are incorporated herein by reference.